It is well known in the art to use laboratory reaction calorimeter devices to obtain design basis data for designing chemical process relief systems. Data obtained include adiabatic rates of temperature and pressure rise for very fast, runaway type reactions. These devices generally operate by heating a test sample contained in a test cell until a threshold of a reaction is detected. Once a reaction is under way, heaters are manipulated to balance heat losses from the test sample so that the sample may remain adiabatic as it reacts.
There are presently available several reaction calorimeters useful for the study of runaway reactions. An example includes the device of Fauske's U.S. Pat. No. 4,670,404. While this device offers general utility, it may tend to be a difficult, expensive, and cumbersome device to operate and maintain due to its relatively complicated configuration. A less expensive, simpler reaction calorimeter useful for obtaining relief system design basis data is described in detail in Fauske's later U.S. Pat. No. 5,229,075, the teachings of which are herein incorporated by reference.
In order to offer a simpler, less expensive, and easier to use system, the device disclosed in the '075 patent utilizes a single heater, single temperature probe configuration, as opposed to a plurality of heaters and temperature probes of prior devices. Although the '075 device satisfied a need for a relatively uncomplicated tool, it presents several problems. Primary among these problems is the mode of heating a test sample.
To run a test, the '075 device simply ramps a sample temperature at a constant temperature rise ramp rate. If a reaction should occur during that temperature ramping, the '075 device will continue to provide background ramping so as to minimize any heat losses from the cell.
By way of example, a sample may be ramped at a rate of 1.degree. C./min to a temperature of 300.degree. C. If a reaction is encountered at 200.degree. C., the reaction heat will cause the sample to heat faster than the prescribed 1.degree. C./min. In order to insure that heat is not being lost from the sample, the '075 device will continue to input the constant 1.degree. C./min. ramp rate. Thus the actual heat rates observed and measured are a combination of the reaction heat and the device ramping heat. To obtain actual heat rates due to reaction energy, the device ramp rate must be subtracted out of the observed rate data. Such a subtraction disadvantageously introduces numerous approximations, calculations, and associated uncertainty to data interpretation.
In addition to rate data, the onset temperature of a reaction at which an exotherm begins to occur is of great importance to relief system designers and others. Because of its simple mode of operation, the '075 device must heat a sample at a constant ramp rate to search for an exothermic reaction. Its method of heater control is not capable of holding a sample in an adiabatic state to search for an exotherm. Because the '075 device is heating a sample at a constant ramp rate, reaction heat will not be evident until that reaction heat is substantial enough to cause the temperature rise rate to exceed the background ramp rate.
For instance, if a sample is being heated at an imposed rate of 1.degree. C./min. by the device heater, an exotherm that may occur will not be evident until it causes the observed rate to rise some amount over 1.degree. C./min. By this time, however, the reaction has been under way for some time. The heating control scheme of the '075 device therefor causes a lack of sensitivity in detection of reaction onset. In order to estimate an onset of reaction temperature when using this heating control scheme, it is necessary to subtract out the background imposed ramp rate from the observed heat rate. Such a data treatment requires several approximations, calculations, and introduces uncertainty.
Other calorimeters are capable of heating a sample such that observed temperature rise rates are due only to reaction heat, and are able to hold samples in an adiabatic state to detect exothermic reaction onset at very low levels. Prior art devices that have these capabilities, however, require a relatively expensive, complicated design with a plurality of heaters and temperature probes. No prior art devices have been able to achieve satisfactory heater control for accurate measurement of rates and onsets in combination with a relatively simple and inexpensive general configuration with only a single heater and a single temperature probe.
The method of heater control of prior art calorimeters such as the '075 device that use a single heater and thermocouple also have a problematic manner of heating a sample when a reaction is not occurring. These devices calculate the amount of heater power to apply to ramp a sample based on a stored calibration algorithm that relates sample temperature to heater power. This calibration algorithm assumes a sample mass, specific heat, and heat loss model. Sample mass may be somewhat predictable and controllable by a user. Sample specific heat, however, is very unpredictable. For a typical organic material, for instance, a specific heat may be expected to be approximately 0.5 cal/(gm .degree. C.), while for an aqueous material the specific heat would be expected to be twice this amount. Further, the heat loss model may vary considerably from test to test, particularly as the test pressure is varied.
These variances often result in the control algorithm of these prior art devices to apply inaccurate amounts of heat, resulting in imposed ramp rates of the sample that can vary greatly from the desired imposed rate. It is not uncommon for heat rates to vary by a factor of 2 or more when using the heater control scheme of the '075 device, for instance. In some cases, the errors resulting from incorrect calibration assumptions may lead to a sample ramp rate that is not constant but increases over time, which may be mistakenly interpreted by a user as a reaction exotherm. Likewise, the ramp rate may decrease over time, potentially masking an exotherm.
In addition to problems with methods of heater controls, an additional problem that all prior art calorimeter devices share is a lack of any means for characterizing the flow regime of a material. In particular, the flow regime of a material under given reaction conditions may be generally characterized as foamy or non-foamy. As its name suggests, foamy system behavior is generally characterized as a tendency for the liquid level to swell or foam as a reaction occurs and vapor or gas is generated in a liquid bulk. A common example of foamy behavior would be soapy water as air is blown into it; a great deal of foam results. A non-foamy system, on the other hand, does not tend to produce significant liquid level swell or foam during a runaway excursion. Water without any soap additives, for instance, does not foam appreciably as air is blown into it.
No known prior art calorimeter systems or other bench scale systems are equipped to make flow regime characterizations, such as a determination of whether a reaction under given conditions may be characterized as foamy or non-foamy. Further, it is not possible to predict whether a material may be characterized as foamy or non-foamy when under runaway reaction conditions based on physical property data alone. Currently, the only method by which flow regime characterization such as foamy or non-foamy classification may be made is through visual observation. As this practice is not safe or practical for a reaction under runaway conditions, observation is not useful means of obtaining relief system design basis data.
In terms of relief system design, the characterization of a system as foamy or non-foamy is of critical importance. A foamy system presents a much more challenging system to accommodate under runaway conditions than does a non-foamy system. A foamy system generally requires larger overall capacity, with larger diameter vent piping and larger capacity down stream relief system components. Without such accommodations foamy systems may result in pressure rises that exceed vessel design pressures and cause vessel failure. As there is presently no known available practical method or apparatus for determining whether a reactive system is foamy or non-foamy, current relief system design practice is to generally assume all systems are foamy and to thus design overly conservative relief systems in many cases.
Further, for a given foamy system, there are no calorimeter devices capable of determining at what point during a reaction foamy behavior begins. Such information would be of great value, as a relief system could potentially be designed to accommodate the reaction during its non-foamy stage, thereby resulting in a less extensive, less costly system.
In conclusion, an unresolved need in industry exists for a method and apparatus for characterizing a material's flow regime characteristics under runaway conditions.
Further, there is an unresolved need for a reaction calorimeter useful for obtaining relief system design basis data that combines a relatively simple, low cost design with a reliable method of heater control.
There is also an unresolved need for a simple reaction calorimeter useful for obtaining relief system design basis data that offers consistent and accurate imposed heat rates.
There is also an unresolved need for a calorimeter tool which uses a relatively simple, single heater and single temperature probe configuration that does not require searching for an exothermic onset temperature during ramping.